"What Counts As A Planet And A Satellite?"
- A planet is a large natural object in space that moves around the Sun along a fixed path.
- This path is not random; it follows a predictable route controlled by gravity.
- Planets do not produce their own light but can be seen because they reflect light from the Sun.
- The motion of planets helps scientists identify and classify them.
The IAU Definition Of A Planet (2006)
Planet (IAU definition)
An object that orbits the Sun, is massive enough to be nearly round due to its own gravity, has cleared the neighborhood around its orbit, and is not massive enough to sustain nuclear fusion.
- In 2006, the International Astronomical Union (IAU) defined a planet in our solar system as an object that:
- is not so massive that it starts fusion (otherwise it would become a star),
- orbits the Sun,
- is sufficiently massive that gravity makes it nearly round,
- has cleared the neighborhood around its orbit.
- The last condition explains why Pluto, Ceres, and Eris are classified as dwarf planets rather than full planets: they share their orbital region with many other objects.
Dwarf planet
A solar-system object that orbits the Sun and is nearly round due to its own gravity, but has not cleared the neighborhood around its orbit.
The Solar System Has Eight Planets With Very Different Properties
- In the modern view, the solar system contains eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- They can be grouped into:
- Terrestrial (rocky) planets: Mercury, Venus, Earth, Mars (smaller, denser, solid surfaces)
- Giant planets: Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants)
- A key pattern is that distances between planets increase strongly as you move outward.
Outer planets like Uranus and Neptune were unknown to ancient astronomers because they are much dimmer and move slowly against the background stars.
- In the MYP eAssessment N23, a drag-and-drop question tested knowledge of the correct order of planets from the Sun.
- The key requirement is recalling the correct sequence: Mercury, Venus, Earth, Mars, followed by the gas giants.
Approximate Sizes And Distances
The table below summarizes typical distances from the Sun and planetary radii.
| Object | Distance From Sun (million km) | Radius (km) |
|---|---|---|
| Sun | - | 695,700 |
| Mercury | 57.91 | 2,440 |
| Venus | 108.2 | 6,052 |
| Earth | 149.6 | 6,371 |
| Mars | 227.9 | 3,390 |
| Jupiter | 778.5 | 69,911 |
| Saturn | 1,429 | 58,232 |
| Uranus | 2,877 | 25,362 |
| Neptune | 4,498 | 24,622 |
Use ratios to compare. Jupiter's radius is about $69\,911/6\,371 \approx 11$ Earth radii, while the Sun's radius is about $695\,700/6\,371 \approx 109$ Earth radii.
Orbits and Planetary Motion
Orbit
An orbit is a circular or elliptical path followed by an object as it moves around another object in space.
How do planets move around the Sun?
- Planets move along paths called orbits.
- An orbit is usually elliptical, meaning it is slightly stretched rather than perfectly circular.
- The Sun lies at one focus of the elliptical orbit.
- Gravity between the planet and the Sun keeps the planet in its orbit.
Why Are Some Objects Not Called Planets? (Dwarf Planets)
Dwarf planet
A solar-system object that orbits the Sun and is nearly round due to its own gravity, but has not cleared the neighborhood around its orbit.
- Not all objects that orbit the Sun are classified as planets.
- Some objects are large and spherical but share their orbital region with many other objects.
- These objects are called dwarf planets.
Pluto was reclassified as a dwarf planet after the discovery of Eris, which is a similar size.
Gas Giants
- Gas giants are very large planets.
- They are mostly made of gases rather than solid rock.
- They may have a small solid core at the centre.
Gas giants are usually found farther from the Sun than rocky planets.
Planetary Size and Distance Patterns
- Planet sizes increase dramatically from Earth-sized planets to Jupiter.
- Distances between planets increase much faster than their sizes.
- This creates major challenges when trying to draw or model the Solar System.
Using Ratios to Compare Sizes
- Jupiter’s radius is about 11 times Earth’s radius.
- The Sun’s radius is about 109 times Earth’s radius.
- These ratios help compare objects without memorising exact values.
When comparing sizes or distances, use ratios, not absolute numbers.
Scale Models and Why Space Is Mostly Empty
- A scale model keeps all sizes and distances in the same ratio.
- In the Solar System, sizes and distances differ by enormous factors.
- If planets are large enough to see, distances become extremely large.
- If distances fit on a page, planets become tiny dots.
- If Earth’s diameter is reduced to 1 mm:
- The Sun becomes about 11 cm wide.
- Earth would be about 12 m from the Sun.
- Neptune would be hundreds of metres away.
What Is a Satellite?
Satellite
A natural object that orbits a larger body due to gravity, such as the Moon orbiting Earth.
- A satellite is an object that orbits another object.
- A natural satellite is not made by humans.
- Most planets have at least one natural satellite.
The Moon is Earth’s natural satellite.
Motion of Satellites
- Satellites move in orbits due to gravitational attraction.
- The satellite’s forward motion and gravity together create a stable orbit.
- Satellite orbits can be circular or elliptical.
Eclipses and Satellites
Lunar eclipse
A lunar eclipse occurs when the Earth is between the Sun and the Moon.
- A lunar eclipse occurs when Earth moves between the Sun and the Moon.
- Earth blocks sunlight, causing its shadow to fall on the Moon.
- The curved edge of Earth’s shadow provides evidence that Earth is spherical.
Exoplanets (Planets Beyond the Solar System)
Exoplanet
An exoplanet is a planet that orbits a star outside the Solar System.
- Not all planets orbit the Sun.
- Planets that orbit other stars are called exoplanets.
- Exoplanets are detected using indirect methods, such as changes in starlight.
Planetary Size and Distance Patterns
- Planet sizes increase dramatically from Earth-sized planets to Jupiter.
- Distances between planets increase much faster than their sizes.
- This creates major challenges when trying to draw or model the Solar System.
How Gravity Shapes Orbits And Planetary Motion
- Planets orbit because of the gravitational attraction between the Sun and each planet.
- Gravity provides the centripetal force needed to continually change the direction of the planet's velocity, producing a curved path.
- Even without heavy mathematics, connect these ideas:
- Stronger gravity (closer to the Sun) generally leads to faster orbital motion.
- Larger orbital radius generally means a longer period (years are longer farther from the Sun).
- Elliptical orbits mean distance (and speed) can vary during the orbit.
- State the definition of a planet used in MYP physics.
- Explain what is meant by an orbit.
- Describe the difference between a planet and a dwarf planet.
- Explain why Pluto is no longer classified as a planet.
- Define a natural satellite and give one example.
- Explain why Solar System diagrams are usually not to scale.
- State one reason why gas giants are different from rocky planets.
- Explain what an exoplanet is.